Thermostatic metal



Feb. 19, 1935. c. R. E. WOHRMAN 1,991,438

THERMOSTAT IC METAL Filed Dec. 9, 1931 (ALL. x1o'). 16 18 2o 22 24 26 2aCOEFFICIENT 0F THERMALEXPANfiION 2 4 6 a 10 TEMPERATURE (ocenessCENTIGRADEL,

Patented Feb. 19, .1935

. UNITED STATES THERMOSTATIC METAL Carl R. E. Wohrman, 'Attlcboro,Mass., assignor to General Plate Company, Attleboro, Mass., atcorporation of Massachusetts Application December 9, 1931, Serial No.579,922 7 Claims; (01. 297-15) This invention relates to thermostaticmetal, and with regard to certain more specific features,

to thermostatic metal particularly adapted for.

use under corrosive conditions, and/or at high temperatures.

Among the several objects of the invention may be noted the provision ofthermostatic metal of the class indicated which is capable ofsuccessfully withstanding the attack of corrosive agencies, both liquidand gaseous, especially at intermediate temperatures (300-900 F.) andhigh temperatures (900 F. upwards) the provision of thermostatic metalof the class described which is capable of successful operation attemperatures in excess of 1000 F.; and the provision of thermostatic.metal of the class described which is readily formed from inexpensive,readily available material. Other objects will be in part obvious and inpart pointed out hereinafter.

The invention accordingly comprises the elements and combinations ofelements, features of construction and composition, and arrangements ofparts, which will be exemplified in the structures and productshereinafter described, and the scope of the application of which will beindicated in the following claims.

The accompanying drawing is a graph illustrating thetemperature-expansion characteristics of certain alloys.

While a great variety of thermostatic metals have been and now are inuse in temperature con-'- trolled devices, up to the present time noneofthe metals have been capable of successfully withstanding the attack ofcorrosive agencies generally. Attempts have been made to protectthermostatic metal elementsby electroplating them, or otherwise platingthem, with brass or a similar corrosion-"resistant medium. However, theporosity of such plating is objectionable; also, the fact that eachelement must be individually plated and polished makes such proceduresquite impracticable. v Welded or soldered plating ordinarily leaves theedges of the element unprotected, and even if the edges are protected,piercing of the element as for mounting exposes the region around thepiercing to attack. Further, platings of the above characteristics provea deterrent to proper functioning of the element, and objectionablydecrease its sensitivity.

Further, there has been up to the present time no thermostatic metalwhich operates successfully and consistently at temperatures in excessof 1000 F., retaining its elasticity at such temperatures and being atthe same time free from,destructive oxidation and scaling.

The present invention provides thermostatic metal which is adapted foruse at the high temperatures in question, and which also successiullywithstands the attack of such corrosive agencies,

thereby overcoming both of the above dlsadvan tages.

' While the present invention will be described in connection withthe-more usual thermostatic bimetal, comprising a composite sheet havingtwo layers, each layer having a different temperature coeflicient ofexpansion, it is to be understood that the invention is likewiseapplicable to trimetals,

or any multimetal or like forms. i

Proceeding with the description of the present invention as applied moreparticularly to thermostatic bimetal, the individual metal layers willbe considered.

The high expansion metal layer according to the present inventioncomprises a 'nickel-chrd mium steel of the stainless or heat-resistingvariety. The range of composition of such nickelchromium steel isapproximately as follows:

' Per cent Ni 7-40 Cr 14-40 C 0-3 Mn 0-3 W 0-15 P below 0.05 S below 0.05 Fe "balance Phosphorus and sulphur are unavoidably pres-. ent insmall amounts as impurities.

Molybdenum, silicon, cobalt, and vanadium may be added in amounts up tothe order of 10% of each to improve the quality of the metal for specialin stances.

Nickel-chromium steels of the class indicated are characterized by theirhigh resistanceto corrosion at ordinary as well as at elevatedtemperatures. This property permits the use of a thermostatic metal madefrom such nickel-chromium steels in corrosive media such as ammonia,sulphur-bearing fumes, acid and alkaline liquors, corrosive waters andoils, and sundry organic liquors and gases and the like. Thesenickelchromium steels have at the same time an abnormally highresistance to oxidation and scaling ,at high temperatures.

Nickel-chromium steels of the class described have a temperaturecoeflicient. of expansion of about 16 x 10- per degree C. forthetempera-' ture interval 0 to 200 C., and of about 20 x 10- for thetemperature interval 0 to 1000 C. (See shaded area A of the drawing).Thus the coeflicient of thermal expansion of such steels comparesfavorably with that'of the Monel metal now extensively used inthermostatic metals in the intermediate temperature field.

} These nickel-chromium steels are adapted particularly for use as thehigher expansion member of the thermostatic metal, the particularanalysis (and heat treatment) being varied for specific applicationswithin the approximate limits indicated. As a non-limiting example, onesuitable composition for use with low and inter- Another non-limitingexample, being a suitable composition for high temperature (900 to 1500)thermostatic metal is as follows:

Per cent N1 Q Cr 17 Mn 0.6 Si s 3 W i .15 C below 0.15 S below 0.04 Pbelow 0.04

Fe balance,

The nickel-chromium steels above described may be used with any suitablelower expansion member or members. For high temperature use, thestainless chromium irons hereinafter described are recommended. For lowand intermediate temperatures, the currently employed nickel irons ofthe invar group containing some 36% to 48% nickel may be used, and, ifnecessary,

. of corrosion in question.

It will be understood that in many cases only one side of a thermostaticmetal need be corrosion resistant, as in installations where thethermostat is mounted in the walls of the vessel and only one face isexposed to the corrosive medium. In such instances, the nickel-chromiumsteels described are desirable as the component metal coming in contactwith the corrosive media, and it will beseen that any other suitablemetal may be used as the other component of the thermostatic metal, itnot being necessary for it to have, under such conditions, the qualityof resisting corrosion.

The recommended metal for use as the lower expansion component in a hightemperature thermostat hereinbefore indicated as chromium iron,comprises an alloy of that general description containing in excess of14% chromium. The range of composition of such chromium iron issubstantially as follows: I

Per cent lCr 14-30 Mn 0.3-0.7 C 02.5

W 0-15 P below 0.05 S below 0.05 Fe balance Molybdenum, silicon, cobalt,vanadium, and

um steel described, a suitable composition is as follows: I

Per cent Cr 1'? Mn 0.3 Si 0.8 C 0.06 P below 0.04 S below 0.04 Febalance Another non-limiting example, adapted particularly for use .withthe second nickel-chromium steel described, is as follows:

Chromium irons of the class indicated are characterized by theirresistance to corrosion by a great variety of corrosive agencies atordinary as well as at elevated temperatures and are able to standoccasional temperatures up to 1100 C. without defective oxidation orscaling. The chromium content is varied in accordance with the maximumtemperature of which it is desired to use. A higher chromium contentendows the alloy with a higher resistivity to corrosion at hightemperatures. Conversely, a lower chromium content is sufficient inalloys which are to be used at lower temperatures.

Because the temperature coefiicient of expansion of chromium irons ofthe class described is about 10x10 for the interval 0-200 C. and about12 x 10 for the temperature interval 01000 C. (See shaded area B of thedrawing.) It will be seen that this coefiicient is considerably lowerthan that of the nickel chromium steel hereinbefore described, and it isthus a suitable metal to plate with the nickel chromium steel to form athermostatic bimetal.

This temperature coefficient of expansion, however, is considerablyhigher than the temperature coeflicient of expansion of invar or 42%nickel steel now extensively used, in the low and intermediatetemperature fields, but is, however, lower than the coefficient of thesemetals at temperatures above the order of 350 C. Line C of the drawingrepresents a typical 42% nickel steel. Chromium irons, accordingly, arepreferable even to the invar or 42% nickel steel for thermostatic metalsoperating at 350C. or higher, even when the highly desirable corrosionresistivity characteristic is left out of consideration.

The higher expansivity of the chromium irons for temperatures up to 350C. has the advantage of preventing the building up of unnecessarystresses in elements intended for use at, say, 500 C. and higher. Inaddition, there is sufiicient deflection at lower temperatures to makethe metal useful .in elements adapted for such lower temperatures,especially in thermostats of a. devel-= opable character in which smalldeflections and stresses are desired in order to prolong the life of theelement.

While the chromium irons above described are used primarily as the lowerexpansion member in conjunction with the nickel chromium steelshereinbefore described, they are likewise adapted for advantageous usewith other high expansion members, such as Monel metal. nichrome, and

without departing from the scope of the invention, it is intended that.all matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

I claim:

l. Thermostatic metal comprising a plurality oi layers, the higherexpansionof said layers having a composition substantially as follows:

Per cent Ni 8-9 Cr 18-19 n o6 Si 0.5 W--- 0.1 Mo--- 3 C below 0.05 Sbelow 0.04 P below 0.0% F balance and the lower expansions? saicllayershaving a composition substantially as follows:

Per cent Cr 1'? Mn 0.3 Si 0.3 Q 6.06 v P below 0.04

S. below 0.04 F balance 2. Thermostatic, metal comprising a plurality oflays the higher expansion of said layers havmg a composition substtiallyas follows:

Percent Ni 25 Cr 17 m 6.6 Si 3 W I 0.15 C below 0.15 8 4 belowiiili Pbelow 9.04 F balance and the lower exsoisi i or said layers having acomposition sritia v as follows:

Per t Cr "r so Mn 0.3 El--- :3

. 0---- 0.07 W 0.06 P I below 0.04 s below 0.0% F balance 3.Thermostatic metal comprising a plurality of layers, the highercoefiicient of expansion layer having a composition substantially asfollows:

Per cent N 8-9 Cr 18-19 Fe "balance and the lower coemcient of expansionlayer having a composition substantially as follows:

Percent Cr 1! F balance 4. Thermostatic metal comprising a plurality oflayers, the higher coefficient or expansion layer having a compositionsubstantially as follows:

Per cent Ni "1-40 Cr. 14-40 Fe balance and the lower coemcient ofexpansion layer having a composition substantially as iollowsz Fer centCr; 7 1 lei-30 Fe balance 5. Thermostatic metal as set forth in claim 4.in which the composition of the higher coemcient of expansion layer issubstantially as follows:

Per cent. Ni 8-9 Cr 18-19 Mn- 0.6 Si 0.5 W- 0.1 Mol 3 c l... below oneS- .4 below 9.0% P below cos Fe balance 6. Thermostatic metal as setforth in claim 4. in which the composition of the higher coeficient ofexpansion layer is substantie. as iollows:

. For cent Ni. 25 Cr 17% Mn- 0.6 S 3 W 0.15 G below $.15 S- below are P;below 0.0% F balance 7. 'i'lieostatic metal as set forth in. claim 4, inwhich the composition of the lower coefilcient oi espion layer issubstantially as follows:

Per cent

